The police have had problems identifying the victim, believed to be in his late 60s, who is in intensive care in a critical condition after the attack. Police spoke to local people and distributed letters appealing for help.

Late Wednesday, Scotland Yard said officers now believed they know his identity and were trying to trace his next of kin.

He had tried to stop youths from setting light to two industrial bins on Springbridge Road, near the Arcadia shopping centre, at about 10.45pm when rioting was at its height in Ealing.

Police say he was trying to stamp out the fire when he was set upon by a mob, and was punched and fell to the ground, fracturing his skull. He was taken by ambulance to a west London hospital where he remains. He had no wallet or identification on him.

The suspect, whose image on CCTV has been released, is a black man with a big build, who is seen wearing a white T-shirt with writing on the front and a dark-coloured jumper over his shoulders.

Detective Chief Inspector John McFarlane, who is leading the inquiry, said: "Through CCTV we have identified a strong suspect. I know that, on seeing these images of him, people will be able to identify him. If you know who this man is, please contact my incident room. If you are this man in the CCTV, do the decent thing and give yourself up."

Pictures were released earlier of the victim lying face down on the pavement, being helped by a police officer and an unidentified man in a red shirt.

Ealing residents expressed sadness over the attack. On Twitter, he was being hailed as a "hero of the community". "I don't recognise either man [but] it would be nice to tell him 'we appreciate your help'," said Samandar Samari, manager of the Caspian hotel just doors down from Springbridge Road.

Jim Martin, the owner of several Ealing businesses including Big Jim's Trims barbershop, said he had witnessed the aftermath of the attack but had not seen the victim's face. "I was there at about 11pm, and by the time I got there two police officers were trying to help the guy on the ground," he said. "I later found out it was because he had tried to put out the fire and the guys had jumped him and mugged him."

Although he did not recognise him at the time, Martin said he had spoken to police and now believed the man was a customer whose name he did not know.

"He was a really, really nice guy, a nice, old, public school gent, really nice, wouldn't harm a fly," said Martin.

McFarlane said: "I need the assistance of the community who may have witnessed the attack on this innocent man to come forward and provide information or images they may have recorded on mobile devices."

Anyone with information is urged to contact the incident room on 0208 721 4205 or Crimestoppers on 0800 555 111.